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Formula: R.P.M, Ratios, Speed
#1
Quick formula for working out Revs Per Minute and Ratios to Road Speed.

eg: at 7,000 Rpm in top gear road speed of 300km/h

First we need to know the tyre circumference in meters:

eg: 275/40 x 20 Tyre and Wheel

work out wheel and tyre diameter first

 Tyre width = 275mm
 Tyre Profile = 40% x 275mm = 110mm
110mm top and bottom of the wheel =220mm
 + the Wheel diameter
(20 inches wheel dia x 25.4 = 508mm)

220 + 508 = 728mm Total wheel diameter.

x Pi (3.14159) to get the circumference = 2287mm divded by 1000 to get to meters = 2.287 meters of tyre circumference on a 275/40 x 20 tyre.
2.287m 

Now the rest is simple.

RPM / Gear Ratio / Diff Ratio x Tyre Circumference x 60 / 1000 = Km/h

RPM divided by Gear Ratio divided by Diff Ratio multiplied by Tyre circumference multiplied by 60 divided by 1000 = Km/h

eg: 2200 rpm / 0.76 gear ratio / 4.10 diff ratio x 2.288 tyre circumference x 60 time / 1000 distance = 96.9 Km/h or
      1000 rpm / 0.76 gear ratio / 4.10 diff ratio x 2.288 tyre circumference x 60 time / 1000 distance = 44 Km/h x 2.2 for 2200 rpm = 96.9

I hope you like this formula, its good for working out speedo change with tyre size change or for racing purposes.
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